1. Field of the Invention
The present invention relates to a sidestream respiratory gas sampling system with flow based feedback control, and, in particular, such a sidestream sampling system that utilizes a pressure drop across a capillary tube to measure and control the sidestream flow.
2. Description of the Related Art
Gas analyzers are widely used in medical applications and are typically categorized into two different types: (1) “diverting” or “sidestream” gas sampling systems; and (2) “non-diverting” or “mainstream” gas sampling systems. A mainstream gas sampling system includes a sample cell that is disposed along the main path of a breathing circuit through which a patient's respiratory gases flow. As a result, the patient's inspired and expired respiratory gases pass through a sample cell, which is also known as a “cuvette”. A gas measurement system, which includes the elements necessary for monitoring respiratory gases, such as a radiation source and detector, is coupled to the sample cell to measure the constituents of gas passing through the sample cell. An example of such a conventional mainstream gas measurement system is shown in U.S. Pat. No. 4,914,720 to Knodle et al.
A sidestream type of gas sampling system transports a portion of sampled gases from the sampling site, which is typically a breathing circuit coupled to the patient's airway or directly at the patient's airway, through a sampling tube to the sample cell, where the constituents of the gas are measured by a gas sensing system. Gases are continuously aspirated from the sample site, through the sampling tube, and into the sample cell, which is located within a gas measurement instrument. To obtain good quality capnographic waveforms and accurate CO2 measurements, the gas stream flow rate must be well regulated using a flow controller, such as a closed loop or feedback flow control system.
Flow control systems known in the art utilize an orifice to generate a pressure drop to measure the flow rate. The speed of a pump used to draw gas to the sample site is adjusted to maintain a constant gas stream flow rate through the sample site. The pressure drop across the orifice is affected by the density of the gas. As a result, the pressure drop that occurs across the orifice is altitude dependent and is affected significantly by the composition of the gas. The volumetric flow (Q) through an orifice is derived using Bernoulli's principle, and can be easily shown as:
                              Q          =                      K            ⁢                                                            2                  *                  Δ                  ⁢                                                                          ⁢                  P                                ρ                                                    ,                            (        1        )            where K is a constant derived from the orifice dimensions, ΔP is the pressure drop across the orifice, and ρ is the density of the gas. It can thus be appreciated that the density of the gas plays a major role in volumetric flow measurements using an orifice.
Gas density is proportional to pressure, and atmospheric pressure is inversely proportional to altitude. Using an orifice to regulate flow can result in an increase in flow rate as gas density decreases, such as at increasing altitude, i.e., the pressure decreases. Similarly, gas density may change significantly from nominal conditions due to the administration of therapeutic gas mixtures, such as helium and oxygen, to the flow of gas being delivered to the patient. Gas stream flow rates in known sidestream systems range from about 50 ml/min to about 250 ml/min. Examples of conventional sidestream gas sampling systems are taught in U.S. Pat. Nos. 4,692,621 to Passaro et al.; 4,177,381 to McClatchie; 5,282,473 to Braig et al.; and 5,932,877 also issued to Braig et al.
Conventionally, the sampling ports used by sidestream gas sampling systems are located in a wall of the respiratory circuit or an airway adapter. The location of the sampling port along a breathing circuit may range anywhere from an elbow connected to an endotracheal tube, to a wye (Y) connector at the opposite end of a breathing circuit. For example, the sampling port may be placed on the ventilator side of an in-line filter or heat-moisture exchanger (HME). This results in a drier sampling tube but with the inherent risk of significant distortion of the capnographic waveform and lower end-tidal values.
It is also well known in the art to locate the sampling port on the patient side of the in-line filter. However, there is a possibility of an accumulation of condensate and/or patient secretions in this configuration for a sidestream sampling system. Condensation from a humidified sample gas, in combination with patient secretions, can block and contaminate the sampling tube, which may necessitate frequent replacement. The effectiveness of water traps and water filters vary between manufacturers, but no water trap or water filter is immune to eventual clogging and distortion of the capnographic waveform, particularly if preventive maintenance is inadequate. Additionally, water and/or contaminants may break through the filter, clogging the system at the point of smallest cross-sectional area, namely the orifice used for flow control.
Given these problems with sidestream capnography, it is desirable to provide a sidestream gas sampling flow control system that is (a) less affected by altitude and the composition of the gas, and (b) less likely to become occluded by condensate and/or patient secretions than conventional systems.